Most analog and RF transceivers ICs suffer from tolerances and part-to-part performance variation that require calibration of each device. Product calibration generally entails adjusting one or more tuning parameters whilst observing a particular performance metric and finding the tuning parameters values that yield device performance within the desired range.
Product calibration is typically performed as part of a wider product test implementation. The general setup for such a product test is shown in FIG. 1.
The device under test (DUT) 10 receives test signals 11 from test equipment 12 or transmits signals 13 to test equipment 12. Both DUT 10 and test equipment 12 are typically controlled 15 (including test setup) by a controller PC 14 or other controlling device such as a dedicated processing system which also collects 16 the measured data provided by the test equipment 12 and/or the DUT 10.
In some cases a device being calibrated (DUT 10) comprises a controlling element. Furthermore, it is possible that the device being calibrated comprises one or more separate sub-systems that can mutually act as tested and testing sub-systems. For example, the receiver part of a transceiver IC may test the transmitter and vice-versa. In such cases auto-calibration may be possible and no explicit controller PC 14 or test equipment 12 may be needed. However, conceptually separate parts of the system may be associated as controlling element (e.g. PC 14), test equipment 12 and tested sub-system. All following discussions therefore include such architectures, however they may not all be present.
As is understood, testing and calibration, especially when reliant on external test equipment, adds to the overall product cost. The shorter the overall test and calibration time, the smaller the associated cost.
Typically, all calibration methods follow some kind of successive approximation scheme where starting from a single measurement or a series of measurements, the range of candidate tuning parameters is gradually reduced until the search converges on a set of tuning parameters that meet performance requirements.
In a successive approximation scheme, the controlling PC 14 first instructs the DUT 10 to execute a test using an initial set of tuning parameters and it also instruct the test equipment 12 to collect results based on this initial parameter set. Once measurements have been carried out, the results are returned to the controlling unit 14. The controlling unit then checks if the pre-determined desired performance has been met or selects a new set of tuning parameters for a second measurement cycle if the desired performance has not been met. The new set of tuning parameters is chosen based on the previously obtained measurement values. They are not known a priori.
Such iterative schemes normally converge after a few steps. An example can be found in U.S. Pat. No. 7,369,813. Here, an algorithm is described using an example of the calibration of carrier leakage of a wireless transmitter. Carrier leakage describes an unwanted spectral component at the centre of the transmit channel associated with direct-conversion transmitters. In nearly all implementations, carrier leakage is observed due to small mismatches in the analog and RF circuits performing the up-conversion from baseband signal onto the carrier wave signal. The calibration is conceptually simple. Two tuning parameters (referred to as x and y in the following), are added into the signal path, one to the in-phase signal component and one to the quadrature component as would be understood. One particular combination of the two parameters leads to cancellation with the component originating from circuit imperfections and hence a desired performance.
U.S. Pat. No. 7,369,813 proposes a way to quickly converge on the optimum set of tuning parameters. However, the method is iterative: the parameters chosen in a measurement depend on the choice of previous parameters and the measured carrier leakage.
The disadvantage of such iterative schemes is that the communication between controlling PC 14, test equipment 12 and DUT 10 can dominate the overall calibration time. While the actual measurement of a spectral component can be very fast (e.g. one millisecond), the overall turn-around time between setting up a test, triggering the measurement, collecting the results and processing it can often be much longer (e.g. 100 milliseconds).
A major contributor to calibration time is typically the time taken to setup 15 both test equipment 12 and DUT 10 and to collect results 16. Minimizing the number of setup and data collection cycles reduces calibration time and reduces overall cost.
There is therefore a need to minimise the setup and data collection times.